Pipette devices are used, for example, in the area of molecular biology or medicinal analysis for the transfer of liquids. Special pipette tips are thereby often used which are placed on the pipette device and are intended for one-time use. Such disposable tips are also known by the designation “tip.” The pipette device may be a manual pipette device which merely has one single pipette unit. In the course of progressive automation, which is making its entrance into modern analysis laboratories, automated pipettes or so-called pipette robots having a large number of pipette units arranged in a row or in a matrix are also often used. With such automated pipettes, it is possible to suction samples simultaneously from a large number of vessels and to release them elsewhere.
The pipette tips have been constructed, up to now, mostly with a jacket area, which expands conically with which they are mounted on a correspondingly conically shaped coupling stud of the pipette device or preferably of the individual pipette unit. The pipette tip is thereby pressed onto the coupling stud firmly to establish a frictionally engaged press fit between the pipette tip and the coupling stud. In order to achieve the desired tightness comparatively high pressing forces are required. The material of the pipette tip has a certain elasticity which leads to the pipette tip expanding upon pressing onto the coupling cone. In this way microfissures may be formed in the pipette tip which are a cause of leakage. Also dirt particles on the coupling cone may lead to leakage. Moreover the high pressing forces upon placement of the pipette tip have the disadvantage that for the release of the pipette tip correspondingly high forces have to be applied.
To avoid high pressing forces it has been proposed (see U.S. Pat. No. 5,063,790) that the coupling stud be constructed with an O-sealing ring. The pipette tip is thereby put more loosely over the coupling stud. The O-sealing can then be compressed by means of a squeezing device in such a manner that it expands in its radial direction and produces a frictionally engaged support for the pipette tip. In order to release the pipette tip it is sufficient to disengage the O-sealing ring so that it contracts again, and the frictional engagement between the pipette tip and the O-sealing ring is cancelled. This has the advantage that high forces do not have to be applied on the pipette tip neither during the placement of the pipette tip nor during its release.
If a pipette tip, which is designed in the form of a cone in its area intended for coupling with the coupling stud, is pressed on the coupling cone of the first-mentioned solution or is placed on the coupling stud of U.S. Pat. No. 5,063,790, whereby the O-sealing ring is squeezed, in both cases, the problem arises that the position of the pipette tip relative to the coupling stud cannot be adjusted in a defined manner. In the first case various pressing forces result in the pipette tip being shoved onto the coupling cone to various extents. This relates to the expansion of the pipette tip, which occurs in various degrees, depending on the amount of the pressing forces. In the second case, the O-sealing ring—if it is squeezed—presses against a cone surface of the pipette tip. If, in the beginning, there is still no frictional engagement between the O-sealing ring and the pipette tip, then the radial expansion of the O-sealing ring can lead to a slipping of the pipette tip.
A defined position of the pipette tip relative to the coupling stud is however, of essential importance, particularly for automated pipettes, which have a large number of pipette units. During the suctioning of the liquid, different positioning of the pipette tips of automated pipettes with tens or even several hundreds of pipette units can lead, for example, to some pipette tips being properly immersed in the vessels assigned to them or in depressions of a microtiter plate, whereas other pipette tips remain above the level of the liquid in the vessels or depressions. Likewise, it may happen that individual pipette tips hit the bottom of the vessels or depressions, and in this way their mouth opening is blocked, at least to some extent. The consequence can be an insufficient metering accuracy both in the liquid intake as well as in the liquid release.
Therefore the problem of the invention is to describe a way in which, particularly with automated pipette devices with a large number of pipette units, the metering accuracy can be improved during the transfer of liquids.